1. Field of the Invention
The invention relates to digital communications, and more particularly to spread spectrum communications and related systems and methods.
2. Background
Spread spectrum wireless communications utilize a radio frequency bandwidth greater than the minimum bandwidth required for the transmitted data rate, but many users may simultaneously occupy the bandwidth. Each of the users has a pseudo-random code for xe2x80x9cspreadingxe2x80x9d information to encode it and for xe2x80x9cdespreadingxe2x80x9d (by correlation) the spread spectrum signal for recovery of the corresponding information. FIG. 2 shows a system block diagram, and FIGS. 3a-3b illustrates pseudo-random code plus a QPSK encoder. This multiple access is typically called code division multiple access (CDMA). The pseudo-random code may be an orthogonal (Walsh) code, a pseudo-noise (PN) code, a Gold code, or combinations (modulo-2 additions) of such codes. After despreading the received signal at the correct time instant, the user recovers the corresponding information while the remaining interfering signals appear noise-like. For example, the interim standard IS-95 for such CDMA communications employs channels of 1.25 MHz bandwidth and a code pulse interval (chip) Tc of 0.8138 microsecond with a transmitted symbol (bit) lasting 64 chips. The recent wideband CDMA (WCDMA) proposal employs a 3.84 MHz bandwidth and the CDMA code length applied to each information symbol may vary from 4 chips to 256 chips. The CDMA code for each user is typically produced as the modulo-2 addition of a Walsh code with a pseudo-random code (two pseudo-random codes for QPSK modulation) to improve the noise-like nature of the resulting signal. A cellular system as illustrated in FIG. 4 could employ IS-95 or WCDMA for the air interface between the base station and the mobile user station.
A receiver synchronizes with the transmitter in two steps: code acquisition and code tracking. Code acquisition is an initial search to bring the phase of the receiver""s local code generator to within typically a half chip of the transmitter""s, and code tracking maintains fine alignment of chip boundaries of the incoming and locally generated codes. Conventional code tracking utilizes a delay-lock loop (DLL) or a tau-dither loop (TDL), both of which are based on the well-known early-late gate principle.
In a multipath situation a RAKE receiver has individual demodulators (fingers) tracking separate paths and combines the results to improve signal-to-noise ratio (SNR) according to a method such as maximal ratio combining (MRC) in which the individual detected signals are synchronized and weighted according to their signal strengths. Thus a RAKE receiver typically has a DLL or TDL code tracking loop for each finger together with control circuitry for assigning tracking units to received paths.
A base station detecting many mobile users, each with multipath signals, can improve upon the RAKE detection of each mobile user separately by a joint maximum likelihood detection of all mobile users; this provides optimal detection. However, the computational complexity of the joint maximum likelihood detector precludes practical implementation for situations of more than a few users. Similarly, decorrelating detectors are optimal linear detectors but require large matrix inversions for the case of many users and thus impractical computational complexity.
Interference cancellation attempts to emulate joint maximum likelihood detection with lower computational complexity. In particular, a base station with a RAKE receiver for each mobile user can use estimates of interfering detected other mobile users"" signals to cancel them out from its estimation and thereby increase accuracy of the estimates. The interference cancellation can be sequential or in parallel with iterative estimation. Divsalar et al, Improved Parallel Interference Cancellation for CDMA, 46 IEEE Trans. Comm. 258 (1998) describes a parallel interference cancellation method using partial parallel cancellation at each stage of the iterative estimation.
The present invention provides a spread spectrum system with interference cancellation employing a hybrid of sequential and parallel interference cancellation.
This has advantages including lower computational complexity than comparable parallel cancellation.